This invention relates to loudspeakers incorporated in, for example, televisions or the like and, in particular, to an elongated oval, track-shaped loudspeakers.
While the televisions or the like devices incorporate therein the loudspeakers, slimmed exterior design of the televisions has been overwhelming, and consequently the loudspeakers to be incorporated in the televisions have been also adapted in various ways to be elongated oval to be such as an athletic race-track shape which is mountable in narrow restricted zone due to a demand of space factor.
In conventional loudspeakers of this kind, for example, a pole piece and a magnet constituting magnetic circuit are formed in the form of a thin plate, and the pole piece is formed to be circular in section, while the magnet substantially rectangular in the exterior shape is disposed, in the concrete, on a yoke of the magnetic circuit, a plate is provided on the magnet, and a voice coil wound on a voice coil bobbin is disposed in a magnetic gap defined between the plate and the pole piece. A damper is coupled at its inner periphery to outer periphery of the voice coil bobbin, and at outer periphery to an inner wall of a frame of the loudspeaker.
In order to increase the magnetic flux density at the magnetic gap in such loudspeaker, it is necessary to form the magnet as large as possible in allowable extent of exterior of the loudspeaker, whereas a magnetic saturation is caused to occur when the pole piece is still small in its cross sectional area relative to the magnet made larger, and it is impossible to increase the magnetic flux only by means of the enlargement of the magnet.
More specifically, the magnetic saturation occurs to deteriorate the magnetic efficiency even when the exterior size of the magnet is made as large as possible to the allowable extent of the shape of the loudspeaker frame, so long as the cross sectional area of the pole piece is equivalent to a surface area of a circle having a diameter more than {fraction (1/2.9)} in particular of the minor axis side dimension of the loudspeaker, that is, so long as the cross sectional area of the pole is made to be substantially more than ⅓ with respect to the minor axis side dimension of the loudspeaker, due to the relationship between the magnetic flux density and the permeability of a carbon steel forming the yoke.
When on the other hand the diameter of the pole piece is made larger in order to increase the cross sectional area, the width between both side walls along the major axis of the loudspeaker of elongated shape and outer peripheral walls of the voice coil bobbin opposing both side walls is made narrower, and the damper supporting the bobbin is also caused to be narrower necessarily at the particular position. As the damper becomes narrower, there arises such problems that the damper lowers the compliance to reduce the amplitude, and the loudspeaker is deteriorated in the bass range characteristics and in the tone quality.
As an example of prior art improved in the above respects, a loudspeaker of such arrangement as disclosed in Japanese Utility Model Laid-Open Publication No. 62-143398 has been suggested.
In this known loudspeaker, the pole piece and voice coil are formed to be oblong or elliptic to be elongated in the direction of the major axis of the loudspeaker, so that a certain width of space can be secured between both side walls along the major axis of the loudspeaker and the outer peripheral walls of the voice coil bobbin opposing the edges, a good width can be secured for the damper at its portions disposed in said both side spaces, and a desired tone quality can be attained.
According to the above publication No. 62-143398, however, it is realizable to simply secure the width on both sides along the major axis of the loudspeaker by means of the oblong or elliptic shape of the pole piece, but it is still impossible to provide a technique of attaining the optimum magnetic efficiency.
The present invention has been suggested to overcome the foregoing problems, and its object is to provide a track-shaped loudspeaker capable of securing an enough width of damper portions disposed between both side walls along the major axis of the loudspeaker and opposing outer peripheral walls of voice coil bobbin, for rendering the resonance frequency to be in lower range and for improving the magnetic efficiency.
According to the present invention, the above object can be realized by means of an elongated oval, track-shaped loudspeaker wherein a voice coil wound on a bobbin is disposed in a magnetic gap between a pole piece and a plate on a magnet disposed around the pole piece, an elongated oval track-shaped diaphragm is joined to the voice coil bobbin supported through a damper to a frame, the pole piece is track-shaped to elongate along major axis of the track-shaped diaphragm, the voice coil bobbin in the gap as well as the magnet disposed around the pole piece are track-shaped, the damper supporting the voice coil bobbin is track-shaped at inner and outer peripheries, and a cross sectional area of the track-shaped pole piece is made to be equal to that of a circle of a diameter more than ⅓ of a minor axis side length of the loudspeaker, so that no magnetic saturation will occur even with the magnet made larger and a magnetic efficiency can be elevated. The magnetic efficiency is improved by providing to the pole piece a circular cross sectional area of the diameter substantially more than ⅓ of the minor axis side length a, as will be readily appreciated from the view point of magnetic properties of the yoke member (carbon steel, SIOC or the like) employed.